The intestinal epithelium is a highly organized selective barrier that prevents the entry of luminal bacteria from the GI tract. Peritonitis can lead to impairment of the gut barrier resulting in translocation of pathogens across the intestinal mucosa, leading exacerbation of the systemic inflammation and establishment a vicious cycle of inflammation-inflicted epithelial damage. Understanding the pathogenesis of gut barrier dysfunction in peritonitis, sepsis, etc, is one of the most important goals in surgical research. The mechanisms of inflammatory gut barrier failure are poorly understood. Expression of cyclooxygenase-2 (COX-2) and accumulation of its product prostaglandin E2 (PGE2) are critical events in the intestinal inflammation. PGE2 signals through its cognate receptors (EP1-EP4) to elicit a plethora of responses including increased local blood supply, fever and pain. One of the consequences of PGE2 signaling via EP1 is Ca2+ mobilization from the intracellular stores. It is known that Ca2+ activates myosin light chain kinase (MLCK), leading to MLC phosphorylation and actomyosin contractility. In a variety of epithelial cells, MLC phosphorylation causes disruption of tight junctions (TJ) that seal the cell borders. We have established that PGE2 causes dramatic decrease of transepithelial electric resistance (TEER) in Caco-2 monolayers, and that this effect is mediated by EP1, phospholipase C (PLC), Ca2+, and MLCK. We hypothesize that gut barrier breakdown during peritonitis involves expression of COX-2 in the intestinal epithelium, accumulation of PGE2, and PGE2-induced disruption of TJ via EP1-PLC-Ca2+-MLCK. We propose 3 specific aims: 1. To define the roles of COX-2, PGE2, and EP1 signaling in gut barrier failure during peritonitis. 2. To elucidate the mechanism of COX-2 induction by PGE2. 3. To test COX-2-targeting therapies aimed at protecting the gut barrier during experimental peritonitis. This study will yield considerable insight into the role of COX-2 and PGE2 in the pathogenesis of gut barrier failure and help design novel therapies to maintain gut barrier integrity during systemic inflammation.